In general, to form a coating on a flexible support, one or more coating solutions containing all the necessary ingredients are applied as a thin layer or layers to the moving web. The coated web is subsequently dried. A common way to dry such coated webs is to impinge hot air at high speed in an impingement dryer onto the surface of the coated web through a system of nozzles. In this process, however, the surface of the liquid layers, usually with high water content and low vicosity, tends to become uneven due to incessant pressure disturbances in the drier. For applications requiring good coating homogeneity, such as photographic materials or recording sheets for ink jet printing, the problem may be solved by using coating solutions that contain a thermo-reversible gelling binder such as gelatin. After applying the solution containing the thermo-reversible gelling binder to the web, the coated web is chilled to thicken or gel the applied solutions. The gelled layers are less prone to deformations of their surface due to the impingement of the air in the drier.
It is important that the rate of gelation upon chilling be fast, because otherwise the strength of the gelled binder would not be sufficiently high unless very long chilling times would be used. For a given coating installation, where the length of the chilling zone is fixed, longer chilling times can only be achieved by reducing the coating speed.
Only a few binders show thermo-reversible gelling behavior.
In the case where the coating solutions do not include a thermo-reversible gelling binder, chilling of the coated solutions does usually not improve the robustness of the coating surface against deformations due to the impingement of the air in the drying step.
It would therefore be desirable to provide a method that would allow the coating of solutions not containing a thermo-reversible gelling binder. These coating solutions would have a low viscosity during the coating process and then rapidly thicken or gel once applied to the moving web.
Such a method is to incorporate, using known methods, thickening agents to the coating solutions before their application to the moving web. This method, however, creates additional problems such as poor stability of the coating solutions, it requires the delivery fluids of high viscosity and it induces the formation of coating defects due to precipitation and slug formation. For coating solutions containing polyvinyl alcohol, such known thickening or gelation-promoting agents include boric acid and/or borates.
Patent application EP 634,286 discloses a coating fluid comprising an alumina sol and polyvinyl alcohol as a binder and having boric acid incorporated therein. By coating such a coating fluid onto a support, followed by drying, a recording sheet for ink jet printing may be obtained. This recording sheet has a porous ink-receiving layer consisting of a binder and nanoporous aluminium oxide/hydroxide capable of absorbing and fixing the different dyes in the recording liquids. The coating solution described in this patent application incorporates boric acid and/or borates to prevent cracking of the dried recording sheets for ink jet printing. In such a case, however, the gelation-promoting effect of boric acid and/or borates must be minimized in order to keep the viscosity of the coating fluid stable in time at a level compatible with used coating methods. This is usually achieved by dilution of the coating solution with a solvent, preferably water. The gelation-promoting effect of boric acid and/or borates in a coating liquid such as that described in patent application EP 634,286 is thus a drawback during the coating process. This effect would, however, be highly desirable on the coated moving web in order to accelerate the gelation of the applied coating solution and thus eliminate or reduce the number of coating defects.
Patent application GB 2,132,784 describes the use of an overcoat comprising a mixture of polyvinyl alcohol and boric acid together with an inorganic pigment. The application does not disclose a method for controlling the gelation of the coating solution by removing the boric acid from the coating solution containing the polyvinyl alcohol.
U.S. Pat. No. 4,877,686 describes the preparation of recording sheets for ink jet printing, wherein the ink-receiving layer contains polyvinyl alcohol as a binder, boric acid and a filler having a high absorption capacity, and where the ratio of binder to filler is from 10% to 100% by weight relative to the amount of filler.
U.S. Pat. No. 5,034,249 describes the preparation of photographic materials, wherein the coating solution contains an instant hardener, but practically no binder, and where the gelatin containing layers below this coating layer are cross-linked by the instant hardener.
Patent application EP 1,111,452 describes a method for coating a moving web, wherein the web, optionally precoated with one or more layers, is coated in a first step with a coating solution containing a viscosity-increasing ingredient. This coated layer is then dried and subsequently overcoated in a second step with a second coating solution comprising a film-forming polymer to form an image-receiving layer. In a preferred embodiment of the invention for the preparation of recording sheets for ink jet printing, the viscosity-increasing ingredient is a borate and the film-forming binder in the ink-receiving layer is polyvinyl alcohol. It is claimed that a substantial amount of the viscosity-increasing ingredient deposited in the first step solubilizes in the second coating step and diffuses into the ink-receiving layer, where it interacts with the film-forming binder and increases the viscosity of the second coating solution applied to the web in a controlled way. Coating and drying of the first layer containing the viscosity-increasing ingredient is, however, expensive, and it would be highly desirable if this first step could be avoided.
Thus there is a need for an improved coating method without a second coating step, wherein coating defects could be eliminated or their number reduced.